The difference in cost between a good quality hammer and a poor quality hammer is just a few dollars. The shape of the head, the type of the handle, and the weight of the hammer are the most important factors when buying a hammer.
Hammer handles come in ash, hickory, tubular steel with rubber grips, or fiberglass. The head of the hammer should be forged from quality steel. On general-purpose hammers, the surface that comes in contact with the nail should be smooth and have a slightly beveled edge.
Specialized hammers are available for everything from installing drywall to hammering upholstery tacks.